Heretofore, vehicular seat devices have widely been known, which include side supports on both sides of a backrest for restraining a passenger in a vehicle, in order to prevent the passenger from becoming tilted in attitude under centrifugal forces while the vehicle is traveling.
As one type of such a vehicular seat device, a vehicular seat device is disclosed in which portions (protruding portions) of side supports, which project forwardly from a backrest, have widths that are changed in order to enable the extent to which the passenger is restrained to be variable (Japanese Laid-Open Patent Publication No. 06-284942).